What Is a Functional Interface? Functional interfaces are the type system bridge that makes lambda expressions work in Java 8. Without them, the compiler would have no way to know what type to assign to a lambda. Once you understand how the 43 built-in interfaces in java.util.function are organised — and when to write your own — you will find that the same patterns (compose, chain, validate, transform) appear everywhere in a Java 8 codebase.